(1) Field of the Invention
The present invention relates generally to signal simulation, and more particularly to an apparatus for simulating a planar wavefront as it would be received by an imaginary linear array of equally spaced receiving elements.
(2) Description of the Prior Art
Underwater acoustical measurement systems often use a vertical line of equally spaced sensors to receive acoustical signals. This line of sensors is called an array. It is assumed that the acoustical signals are generated at a distance from the array such that they can be considered planar upon reaching the array. These planar acoustical signals are called wavefronts. To increase the array's ability to distinguish between the signal and correlated noise, the sensor outputs are added together to form a spatial filter, otherwise known as an acoustical beam.
Pictorially, the beam forms a cone which starts at the array's center and expands along an axis which is perpendicular to the array's center. Performing in the same manner as a bandpass filter, its electrical counterpart, the cone accepts wavefronts that fall within the cone while ignoring wavefronts outside the cone. This increases the gain or strength of wavefronts (i.e., signals) within the cone relative to signals outside the cone.
A beam whose axis is the same as the array's perpendicular axis can be mathematically represented by stating that the signal outputs from each sensor are "in phase". Two or more signals, of the same frequency, are considered to be "in phase" when they pass through their maximum and minimum points at the same instant. Signals are "out of phase" when they pass through their maximum and minimum points at different points in time. Thus sensor outputs are in phase when the wavefront is parallel to the array. However, if the wavefront arrives at the some angle relative to the array's axis, it is not entirely received by the cone and the array's signal output is reduced. The output is reduced because it takes longer for the signal to travel to each sensor. Therefore, when the output signals from the sensors are added together, the signals are not in phase and the signal strength is reduced.
To correct this situation, a beamformer is usually connected to receive the outputs of the sensors in the array. The beamformer introduces time delays which correlate to phase angles in the mathematical frequency domain. These time delays counteract the delays created by the time it takes the angled wavefront to travel to each sensor thereby forcing all the sensor outputs back in phase. In this way, the signals have their maximums and minimums occurring at the same instance in time so they can be added together and produce the same combined output strength as if the wavefront was parallel to the array's axis and centered at the same depth. This is pictorially interpreted as if the array's beam or cone was steered in the direction of the incoming acoustical wavefront to completely capture the wavefront.
The problem with beamforming systems is that in order to test their performance, a known electrical signal must be injected into the sensors' input amplifiers to simulate an acoustic wavefront. This is straightforward for the parallel wavefront but complications arise when precise incremental delays must be introduced for each sensor's amplifier to simulate a wavefront at some angle to the array's axis.
While discrete components do exist to implement delays in signal lines, these components are configured to create a single delay that cannot be adjusted. Thus, if an array can receive any signal over an angular range of 180.degree., there must be 180 delay line components per sensor channel to simulate all possible wavefronts with one degree of phase accuracy. This is not practical or cost effective.